1. Field of the Invention
This invention relates to fiber-forming, melt-spinnable wholly aromatic copolyesters. The invention includes filaments and films prepared from optically anisotropic melts of the copolyesters. The filaments have high modulus and high tenacity after heat treatment.
2. Description of the Prior Art
Aromatic copolyesters containing units derived from 4-hydroxybenzoic acid, hydroquinone, and isophthalic acid have been disclosed in prior art, e.g., U.S. Pat. No. 3,637,595 and Japanese Patent Application Publication 139,698/79. Relatively high melting points are reported in such prior art. In the case where the units derived from 4-hydroxybenzoic acid constitute 40 mole % or more of the copolyester composition it is found that poor thermal stability at the high temperature and long exposure time required for melt processing lead to poor spinnability and frequent breaks of the filaments in the threadline.
Copolyesters containing units from 4-hydroxybenzoic acid, hydroquinone, isophthalic acid and 2,6-naphthalene dicarboxylic acid are disclosed in U.S. Pat. No. 4,083,829. A copolyester of units from 6-hydroxy-2-naphthoic acid, hydroquinone and terephthalic acid is exemplified in U.S. Pat. No. 4,256,624. The patent also discloses that isophthalic acid may be used, and up to 10 mole % of units from other arylhydroxy acids can be included in the polymer.
Lastly, U.S. Pat. No. 4,219,461 discloses copolyesters containing units derived from 4-hydroxybenzoic acid, 6-hydroxy-2-naphthoic acid, hydroquinone and terephthalic or other diacid having para-positioned chain extending links; it will be noted that relatively large amounts of 6-hydroxy-2-naphthoic acid are employed.